Cystine is a non-essential amino acid formed from the oxidation of two cysteine molecules, and it plays a significant role in the structure and health of hair. Chemically, cystine is a key component of keratin, the primary protein that makes up hair, skin, and nails. The disulfide bonds formed by cystine residues contribute to the strength and rigidity of keratin fibers, which is why cystine (and cysteine) is often included in dietary supplements aimed at supporting hair health.

Scientific studies have shown that adequate intake of cystine and other sulfur-containing amino acids is important for normal hair growth and strength. Some clinical studies suggest that oral supplementation with cystine, often in combination with other nutrients such as B vitamins or millet extract, can improve hair quality, reduce hair loss, and enhance hair growth in individuals with certain types of hair thinning, such as telogen effluvium. However, the evidence base is moderate and often limited by small sample sizes and combination products, making it difficult to isolate the effect of cystine alone.

Overall, there is a plausible biochemical rationale and some supportive clinical evidence for the use of cystine to support the hair body system, particularly in the context of hair growth and strength. Still, more rigorous, large-scale studies are needed to firmly establish its efficacy as a standalone supplement.